1. Field of the Invention
The present invention relates to a process for the production of a color developer. More particularly, the present invention relates to a process for the production of a color developer which reacts with a color former, thereby causing color formation.
2. Description of the Prior Art
A recording sheet is well known which utilizes the color-formation reaction which occurs when a substantially colorless electron donating organic compound such as Malachite Green lactone, Benzoyl Leucomethylene Blue, Crystal Violet lactone, Rhodamine-B-lactam, a 3-dialkylamino-7-dialkylaminofluorane, 3-methyl-2,2-spirobi(benzo[f]chromen), and the like (hereinafter, color former), is contacted with an electron accepting material (hereinafter, color developer).
Representative examples of such recording sheets are a pressure sensitive copying paper as described in, for example, U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,548,366, 2,712,507, 2,730,456, 2,730,457, 3,418,250, etc., and a heat sensitive recording member as described in, for example, Japanese Patent Publication No. 4160/1968 and U.S. Pat. No. 2,939,009, etc., and the like. Furthermore, a printing method is known which comprises transferring an ink containing a color former through a device such as a stencil onto a developer coated sheet (as described in German Patent Application (OLS) No. 1,939,624, etc.).
In many cases, the coloring reaction is caused by the application of a physical action such as pressure, heat, and the like using a pen, a typewriter, or the like.
The most typical example of these recording sheets is a pressure sensitive copying paper. A pressure sensitive copying paper is produced by dissolving a color former in a solvent, dispersing the resulting solution in a binder or encapsulating the resulting solution, and then coating the thus obtained coating solution on a support such as a paper, a synthetic resin film, a synthetic resin coated paper, and the like.
A heat sensitive recording sheet is produced by coating a color former in combination with a heat-meltable material such as acetanilide on a support. In this case, the heat-meltable material is one which melts on heating and dissolves the color former.
On the other hand, a color developer is dissolved or dispersed in water or an organic solvent in combination with a binder, and is coated on or impregnated into the support. The color developer can be coated or impregnated as a composition prior to recording.
In general, the color former and the color developer are coated on the same side of or on both sides of the support, or on different supports.
A solid acid is generally used as the above color developer, and clays such as acid clay, activated clay, attapulgite, zeolite, bentonite, and the like; organic acids such as succinic acid, tannic acid, gallic acid, phenol compounds, and the like; and acid polymers such as phenol resins, e.g., phenol-formaldehyde resins and phenol-acetylene resins, and the like; etc. are known.
Particularly, phenol resins have been attracting public attention as a color developer (as disclosed in Japanese Patent Publication No. 20144/1967), and many improvements therein have been proposed as described in, for example, U.S. Pat. Nos. 3,516,845, 3,540,911, and British Pat. No. 1,065,587.
A phenol resin is excellent in that the color image formed is not affected when contacted with water, but the developing capability of the phenol resin is not sufficient and the light durability of the color image formed is poor. For example, thus color image obtained from a phenol resin and Crystal Violet lactone fades easily when it is allowed to stand under normal room ilumination, not to mention when the image is exposed to sunlight, and the surface of the phenol resin not involved in the formation of the color turns yellow. Furthermore, other acid polymers, for example, a maleic acid-rosin resin, or a partially or completely hydrolyzed styrene-maleic anhydride copolymer are not practical in that their developing capability is inherently low.
Metal compounds of an aromatic carboxylic acid have already been proposed as being effective as a color developer for a recording sheet. Namely, the use of the metal compounds of an aromatic carboxylic acid substantially improves properties of the color developer. However, the developing capability and film strength of the coating layer are insufficient depending upon the preparation of the developer coating solution, and additional improvements have been desired.